The present invention relates to chiral salen catalysts and a process for preparing chiral compounds from racemic epoxides by using them. More particularly, the present invention is to provide chiral salen catalysts and its use for producing chiral compounds such as chiral epoxides and chiral 1,2-diols economically in high yield and high optical purity by performing stereoselective hydrolysis of racemic epoxides, wherein the chiral salen catalyst comprises a cationic cobalt as a center metal of chiral salen ligand and counterions having weak nucleophilic property to resolve disadvantages associated with conventional chiral salen catalysts, and can be used continuously without any activating process of used catalysts because it does not loose a catalytic activity during the reaction process.
Chiral epoxides or chiral 1,2-diols have been widely used to prepare pharmaceuticals and agriculture products having optical properties (U.S. Pat. No. 5,071,868; Tetrahedron Lett., Vol. 28, No. 16, 1783, 1987; J. Org. Chem., Vol. 64, 8741, 1999). Even if these chiral epoxides or chiral 1,2-diols having high optical purity are very useful industrially, use of these compounds has been restricted because the preparation of such compounds is too difficult to produce in a large scale with low manufacturing price.
A preparation method of chiral epichlohydrins as one of chiral expoxides is disclosed using microorganism in EP 431,970 and JP 90-257895 and 94-211822. However, it is not recommended because the productivity is low and it requires two-step process. Another preparation method of chiral epichlohydrins from chiral sulfonyloxyhaloalcohol derivatives obtained from mannitol derivatives is disclosed in U.S. Pat. No. 4,408,063; and J. Org. chem., Vol 43, 4876, 1978. Another preparation method of chiral epichlohydrins from 3-chloro-1,2-propanediol is also disclosed in Syn. Lett No. 12, 1927, 1999. However, these processes are required multi-step syntheses, so that they are also deficient to use for the industrial purpose.
Methods for preparing chiral expoxides generally use a chiral catalyst having stereoselectivity which hydrolyzes stereoselectively only one isomer from racemic epoxides mixed 50 and 50 of each isomer and leaves the un-hydrolyzed isomer in the reaction medium. However, the chiral catalyst used for said stereoselective hydrolysis is usually expensive. Therefore, if it cannot be re-used, it becomes difficult to use for the industrial purpose.
Stereoselective hydrolyses of chiral epoxides using chiral salen catalyst as a chiral catalyst are recently disclosed in Science, Vol. 277, 936, 1997; U.S. Pat. Nos. 5,665,890 and 5,929,232; and WO00/09463 and WO91/14694. It has been reported that the use of chiral salen catalyst provides higher yield with higher optical purity compared to uses of other chiral catalysts. However, it is reported that after hydrolysis of a racemic epoxide using conventional chiral salen catalyst, the product chiral epoxide is racemized as time goes in pages 86-87 of WO00/09463. When this hydrolysis is performed for mass production, the racemization of the product becomes deepened since it takes longer to perform the distillation to obtain the desired product, thus resulting in decrease of optical purity of the chiral epoxide. Therefore, the use of chiral salen catalyst in the production of chiral epoxides is limited for the above-mentioned reasons.
Further, when conventional chiral salen catalysts are reused, it requires an activation process after each use because activities thereof are rapidly decreased. Even if the catalyst is activated after used, the optical activity of the product prepared by using reused catalyst is remarkably lower than that of the product prepared by using fresh catalyst. Thus, there is limited to reuse. Such problems increase the manufacturing price of producing chiral epoxides.
Consequently, demand to produce chiral compounds such as chiral epoxides or chiral 1,2-diols efficiently and economically has been highly increased with the importance of such compounds to prepare pharmaceuticals and agriculture products.
The present invention has been completed by developing novel chiral salen catalyst comprising a cobalt as a center metal and its counterions of PF6- or BF4- to prevent from loosing activities of chiral catalysts and racemization of chiral products because conventional chiral salen catalysts having acetate groups loose their activities or functional groups such as acetate groups thereof.
In other words, it is important to select appropriate counterions bonded to the center metal in chiral salen catalysts used in stereoselective hydrolyses of racemic epoxides. For example, chiral catalysts having nucleophilic groups such as acetate and halogen group as counterions deteriorate optical purity of products and counterions bonded weakly to the center metal in chiral catalysts can be dissociated during the reaction process, resulting in diminished catalytic activity.
The chiral salen catalyst of the present invention not only keeps its activity but also provides excellent production of chiral epoxides without racemization by comprising a cobalt center metal and counterions of PF6- or BF4-. Therefore, an object of the present invention is to provide chiral salen catalysts which keep excellent catalytic activity after used, thus simplifing the manufacturing process since it does not require activation process of the used catalyst and do not contribute for racemization of produced products.
Another object of the present invention is to provide an economical process for preparing chiral epoxides and chiral 1,2-diols from racemic epoxides by using said chiral salen catalyst in high yield and high optical purity.